This invention relates to substituted [1,3]oxazine compounds with a general (i.e., unsubstituted) structural formula described by fused indoline and benzooxazine fragments such that they are fused along the bond connecting positions 1 and 2 of the indoline fragment and the bond connecting positions 2 and 3 of the benzooxazine fragment. These products, as well as processes to make them or to use them, are provided.
Photochromic compounds change their color when illuminated. In most cases, a colorless compound switches to a colored compound upon illumination by light of suitable wavelength and intensity. The photogenerated species reverts to the starting species by either thermal means or further illumination. These reversible chemical transformations are accompanied by pronounced structural and electronic modifications, which often alter the ability of the photochromic compound to emit light. Under these conditions, the photoinduced and reversible interconversion of the colorless and colored states results in the modulation of the fluorescence intensity. Thus, this mechanism of light-induced transformation and reversion can be exploited to regulate the emissive behavior of collections of molecules in solution and even in solids. The investigation of these fascinating systems have led to new light-responsive materials for applications such as ophthalmic lenses for corrective or cosmetic purposes, optical filters to selectively transmit light, optical limiters to nonlinearly decrease their transmittance in response to increased incident light, photonic switches (including routers) to enable optical communication, public or personal displays of text or pictures, and silica or polymeric panes adapted for installation in “smart” windows of homes and buildings.
The term “photochromism” indicates a photoinduced change in color. Rather than the interconversion between two colored states, however, these transformations usually involve a transition from a colorless state to a colored state. Though less common, photoinduced transitions from colored to colorless forms are also possible. Indeed, the definitions “positive photochromism” and “negative photochromism” are often employed to distinguish coloration and decoloration processes, respectively. In any case, a photochromic transformation is always accompanied by profound absorbance changes in the visible region. In fact, visible absorption spectroscopy is the most convenient analytical method to study these processes.
Reversibility is an essential requirement for photochromic transformations. The photoinduced absorbance changes must be reversible by definition. In fact, photochromic compounds can be classified into two broad categories depending on the nature of the reverse process. Both classes share in common the ability to switch from one state to another when irradiated by light. Thermally stable photochromic compounds retain the photogenerated state even after turning off the light source, but return to the starting state after irradiation at a different wavelength. Thermally reversible photochromic compounds, instead, return to the starting state spontaneously when the irradiation is terminated.
Photochromic transformations are generally based on either unimolecular or bimolecular reactions. In most instances, unimolecular photochromic processes involve interconversion between two isomer forms. They can be based on light-induced ring opening/closing, cis/trans isomerizations, or intramolecular proton transfer. Bimolecular photochromic processes are less common. They rely on either the photoinduced cycloaddition of two identical reactants into a single product or on the photoinduced transfer of an electron from a donor to a complementary acceptor.
We have investigated a spiropyran as a photochromic compound, but it suffers at least two major limitations. Its thermal re-isomerization is relatively slow. Thus, restoration of the starting state is delayed by several minutes once the light is turned off and many applications require a quicker response. Furthermore, our spiropyran tolerates only a limited number of switching cycles.
Therefore, it is an objective of the invention to provide an improved class of photochromic compounds that undergo photoinducible [1,3]oxazine ring opening and revert by [1,3]oxazine ring closing. Irradiation of a compound triggers photo-induced cleavage of a [C—O] bond of the [1,3]oxazine ring to form a phenolate chromophore. The photogenerated (colored) isomer may revert thermally to the starting (colorless) isomer. Reversible coloration of transparent or translucent silica or polymeric materials and light-induced switching may involve multiple cycles of interconversion between different colored states. Our invention addresses the need for improved an photochromic compound with: (i) faster switching speed and (ii) better fatigue resistance than spiropyran compounds.
The present invention is directed to improved optical materials and systems for photochromic switching between two different optical states (e.g., a difference in light absorbance). Other advantages and improvements are described below or would be apparent from the disclosure herein.